The 660-km seismic discontinuity, which is a significant structure in the Earth’s mantle, is generally interpreted as the post-spinel transition, as indicated by the decomposition of ringwoodite to bridgmanite + ferropericlase. All precise high-pressure and high-temperature experiments nevertheless report 0.5–2 GPa lower transition pressures than those expected at the discontinuity depth (i.e. 23.4 GPa). These results are inconsistent with the post-spinel transition hypothesis and, therefore, do not support widely accepted models of mantle composition such as the pyrolite and CI chondrite models. Here, we present new experimental data showing post-spinel transition pressures in complete agreement with the 660-km discontinuity depth obtained by high-resolution in situ X-ray diffraction in a large-volume high-pressure apparatus with a tightly controlled sample pressure. These data affirm the applicability of the prevailing mantle models. We infer that the apparently lower pressures reported by previous studies are experimental artefacts due to the pressure drop upon heating. The present results indicate the necessity of reinvestigating the position of mantle mineral phase boundaries previously obtained by in situ X-ray diffraction in high-pressure–temperature apparatuses.

The Earth's mantle is characterised by a sharp seismic discontinuity at a depth of 660-km that can provide insights into deep mantle processes. The discontinuity occurs over only 2km -or a pressure difference of 0.1 GPa -and is thought to result from the post-spinel transition, that is, the decomposition of the mineral ringwoodite to bridgmanite plus ferropericlase. Existing high-pressure-temperature experiments have lacked the pressure control required to test whether such sharpness is the result of isochemical phase relations or chemically distinct upper and lower mantle domains. Here, we obtain the isothermal pressure interval of the Mg-Fe binary post-spinel transition by applying advanced multianvil techniques with in situ X-ray diffraction with help of Mg-Fe partition experiments. It is demonstrated that the interval at mantle compositions and temperatures is only 0.01 GPa, corresponding to 250 m. This interval is indistinguishable from zero at seismic frequencies. These results can explain the discontinuity sharpness and provide new support for whole mantle convection in a chemically homogeneous mantle. The present work suggests that distribution of adiabatic vertical flows between the upper and lower mantles can be mapped based on discontinuity sharpness. Main:The 660-km seismic discontinuity (D660) is the boundary between the upper and lower mantles. Seismological studies based on short-period P-wave reflections (Pʹ660Pʹ-PʹPʹ) have demonstrated that D660 is extremely sharp and less than 2 km thick 1 , which is in striking contrast to the 7-km-thick 410-km discontinuity 1 . Understanding the nature of D660 from a perspective of mineral physics provides important clues to open questions about the structure and dynamic processes in the Earth's mantle, such as slab subduction and upwelling of hot plume.Geochemical studies suggest that the Earth's upper mantle consists of ca. 60% atom-

Goethite, α‐FeOOH, is a major phase among oxidized iron species, commonly called rust. We studied the behavior of iron (III) oxyhydroxide up to 81 GPa and 2100 K using in situ synchrotron single‐crystal X‐ray diffraction. At high pressure‐temperature conditions FeOOH decomposes forming oxygen‐rich fluid and different mixed valence iron oxides (previously known phases of Fe2O3, Fe3O4, Fe5O7, and novel Fe7O10 and Fe6.32O9). Rust is known to form as a byproduct of anoxygenic prokaryote metabolism that took place massively from about 3.8 billion years (Ga) ago until the Great Oxidation Event (GOE) ∼2.2 Ga ago. Rust was buried on the ocean floor and was transported into the mantle as a consequence of plate tectonics (started ∼2.8 Ga ago). Our results suggest that recycling of rust in Earth's mantle contributes to redox conditions of the early Earth and formation of oxygen‐rich atmosphere.